1. Field of the Invention
This invention relates generally to implantable medical devices and relates more particularly to schemes for communicating data between external medical devices and implantable medical devices.
2. Background Information
Recent implantable medical devices such as cardiac stimulators, including pacemakers and defibrillators, are generally controlled by microprocessors. This permits a variety of modes and parameters of the device to be changed by the physician to customize the operation of the device to the patient's condition. Such devices also are capable of sensing a number of physiological parameters of the patient which would be of use to the physician. It is therefore necessary to provide means for communicating with the implantable medical device both prior to and after implantation in the patient. This is often accomplished with an external medical device such as a programmer which provides an appropriate interface to the physician in the form of display and input devices, and which provides a wand that can be placed in proximity to the implantable medical device to provide communication over a short distance.
The communication over distance can be provided by transmission of electromagnetic energy between the external and implantable devices. The information transmitted between the devices can be in the form of discrete pulses of energy that are modulated in such a way as to encode the data to be transferred.
Because it is necessary to maintain the wand of the programmer in proximity to the implantable medical device for the duration of communication, there is a practical limit to the amount of data that can be transmitted within a convenient amount of time. Naturally, the speed of data communications will have a great influence on the total amount of data communicated during a given amount of time. It is therefore desirable to provide a data encoding scheme that permits rapid transfer of data between the devices.
It is a general rule that as the speed of data communication increases, the power consumed to accomplish the communication also increases. This does not present a limitation in the case of the external medical device, since it can be energized from the power mains. In the case of the implantable medical device, however, which is powered by an internal battery, the increased power consumption occasioned by high speed communications can unduly limit the lifespan of the battery. It would therefore be desirable to provide a high speed data communication arrangement that shifts the burden of extra power consumption away from the implantable device to the external device.